Chemical and isotopic analysis of hydrocarbon traces degassed out of Callovo-Oxfordian argilites from Bure (France): methodology and results

Isotopic mass spectrometry online with gas chromatography (GC-C-IRMS) allows the measurement of the carbon isotopic ratios of hydrocarbon gases (C1 to C4) after complete combustion to CO2. The method also permits to measure proportions of the different hydrocarbon gases and CO2. Such data provide valuable information about the source and genesis of a gas sample, as well as the post-formation physicochemical processes which might affect it in the geological environment. In particular, it is possible to distinguish hydrocarbon gases of bacterial or thermogenic origin based on the proportion of methane and the carbon isotope ratio as measured by GC-C-IRMS. However, if the relative concentrations of carbon molecules are very small (from 1 ppm to 1000 ppm), the isotopic information cannot be obtained directly with GC-C-IRMS. An additional step allowing the pre-concentration of hydrocarbons must be introduced in the analytical procedure. Indeed, in order to obtain an adequate accuracy, several operating conditions are of prime importance: sufficient signal intensity, good quality peak shape, low signal/noise ratio. We present here a pre-concentration system based on the removal of major gas compounds (N2, O2, Ar, etc...) and trapping of carbon molecules. The purified carbon molecules are released in an inert carrier gas, focussed through a tiny capillary, and introduced in the chromatograph for separation, combustion of each compound and final analysis in the mass spectrometer. This technique provides the chemical concentration in the initial gas and the carbon isotope composition of each individual carbon molecular compound. Its high sensitivity allows measurement of isotopic ratios for concentrations as low as 1 ppm of methane, 0,5 ppm of ethane and 0,3 ppm of propane, for bulk gas samples of 10 cc in standard conditions. Total analytical uncertainty on δ13C measurements is ± 1 0/00 or better. The results of a first application of this technique to the Callovo-Oxfordian argillite formation, Bure, eastern Paris Basin, are reported here. GC-C-IRMS analysis was performed on samples of gas naturally released from water-saturated argillite cores stored in specifically-designed outgassing cells shortly after drilling. It was possible to quantify the concentrations and δ13C of methane, ethane and propane. (authors)